Feature request: Support synchronous Http.

[kasobol-msft]

The http components of azure-core now support synchronous http e2e.
The ask for autorest is to support generation that creates both synchronous and asynchronous methods at service interface and make calls from client apis fully sync or async (respectively).

Please use the following PRs as blueprints (couple of SDKs with handcrafted code):
Azure/azure-sdk-for-java#30028
Azure/azure-sdk-for-java#30136
Azure/azure-sdk-for-java#30162

Autorest should generate fully-synchronous flows as default if sync clients are requested to be generated.
There should be an opt-in flag to fallback to sync-over-async (i.e. block on async) in case one of the SDKs we generate faces a blocker that needs resolution in the azure-core.

[weidongxu-microsoft]

Possible source of code conflict with CADL dev:

For convenience method in CADL, we might add one additional instance of ClientMethod (or subclass of it) to the current ClientMethod as its class variable, as instruction of what the vanilla method would be, and let the ServiceAsyncClientTemplate provide the implementation of the conversion of vanilla method call to DPG method call.
It will have some changes to the ClientMethodMapper as well.

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Also the sync version of pageable and LRO might be hard, if we use old PagedIterable and SyncPoller in core.

[srnagar]

AutoRest now fully supports sync stack. To enable, add enable-sync-stack: true to the README.md containing the codgen configuration.